Traveling wave tube slow wave structure having bi-filar helices



Jul 6, 1965 Filed June 23, 1961 F. TRAVELING WAVE ".4I'I'I'II 111111!111111 '2 ZAMMAM I I I E. VACCARO TUBE SLOW WAVE STRUCTURE HAVINGBI-FILAR HELICES 2 Sheets-Sheet 1 INV EN TOR. Fed/)4? zwczwea LU. w (MM/m July 6, 1965 Filed Jun 25, 196i E. vAccARo" F TRAVELING WAVE TUBE SLQWWAVE STRUCTURE HAVING BI-FILAR HELICES 2 Sheets-Sheet'Z 0U WM UnitedStates Patent O 3,193,726 TRAVELING WAVE TUBE SLUW WAVE STRUC- TUREHAVING BI-FILAR HELIUES Frank E. Vaccaro, New Brunswick, NJ, asslgnor tothe United States of America as represented by the Secretary of the AirForce Filed June 23, 1961, Ser. No. 119,774 5 Claims. (Cl. SIS-3.6)

This invention, in general, relates to improvements in helical wiretransmission lines as used in beam traveling wave tubes and, moreparticularly, to novel means for suppressing backward wave oscillationin traveling wave tube amplifiers that employ bifilar helices.

Traveling wave tubes usually employ a long, thin cylindrical electronbeam that must be confined continuously throughout its length byelectric and/ or magnetic forces. The common method of overcoming thediverging spacecharge forces in a high density stream of electrons is touse an axial magnetic field, either uniform or alternating, along theaxis of the beam. By using a series of small ceramic permanent magnetsto provide a periodic magnetic field on the axis of the beam, the weightand focusing power required can be reduced substantially below thatnecessary for uniform magnetic fields. However, much of the bulk andweight in a packaged traveling wave tube of this type is still due tothe magnets. The use of electrostatic forces to focus the electron beamin a traveling wave tube eliminates the need for magnet structure,considerably reduces the weight of the packaged tube and also eliminatesthe problem of alignment of the tube within the magnetic field. Becausethe helix used in conventional traveling wave tubes is a relativelysimple structure that makes possible transmission of R-F energy overextremely wide bandwidths, an electrostatic focusing method utilizingsuch a structure is highly desirable. Certain traveling wave tubeamplifiers known in the art provide electrostatic focusing means,wherein the basic R-F properties of the helix are preserved, byinterwinding two helices in a bifilar manner. The electrostatic focusingaction in such a device is obtained by operating the two helices atdifferent potentials.

Electrostatically focused traveling wave amplifiers employing bifilarhelices are, however, subject to the same unwanted backward waveoscillations as are unifilar amplifiers. This condition occurs becauseboth types of amplifier have circuits that will support backward wavemodes. These modes are characterized by the fact that the phase andgroup velocity of the wave are opposite in direction. In generaltraveling wave interaction occurs when an electron beam has a velocitythat is approximately equal to the phase velocity of the wave. It isalso well known that energy propagates along a circuit in the samedirection as the wave group velocity. The fact that backward waves havegroup and phase velocity opposite in direction provide inherent feedbackmechanism and oscillations will build up in these modes when theinteraction between the beam and the fields associated with these modesare of sutlicient strength to overcome the losses in the loop.

Accordingly, it is a principal object of my invention to provide, in atraveling wave tube amplifier, a novel bifilar helical slow wavestructure adapted to suppress oscillations resulting from beaminteraction with the antisymmetric backward wave mode.

It is another object of my invention to provide an electrostaticallyfocused traveling wave tube amplifier including a bifilar helical slowwave structure, said slow wave structure having a geometricconfiguration especially adapted to suppress backward Wave oscillations.

Still another object of my invention is to provide an electrostaticallyfocused traveling wave tube amplifier having two-helical slow wavestructures interwound in a bifilar arrangement, each of said slow wavestructures being fabricated from a difi'erent size wire and havingdifferent electrical potentials impressed thereon.

The novel features which are believed to be characteristic of theinvention, both as to its organization and meth- 0d of operation,together with further objects and advantages thereof, will be betterunderstood from the following description considered in connection withthe accompanying drawings in which one presently preferred embodiment ofmy invention is illustrated by way of example. It is to be expresslyunderstood, however, that the drawings, are for the purpose ofillustration and description only, and are not intended as a definitionof the limits of the invention.

In the drawings:

FIGURE 1 illustrates a traveling-wave tube amplifier of the typecomprehended by my invention;

FIGURE 2 is a sectional view of said traveling-wave tube taken at A-A;

FIGURE 3 illustrates the symmetric forward wave field configuration of aconventional bifilar helix;

FIGURE 4 illustrates the antisymmetric backward wave field configurationof a conventional bifilar helix; and

FIGURE 5 illustrates the antisymmetric backward wave field configurationassociated with the novel structure of my invention.

It is the usual practice to operate traveling wave-tube amplifiers thatemploy bifilar helices in the symmetric forward wave (111:0) mode. Thismode exhibits the R-F field configuration illustrated in FIGURE 3. Thebeam interaction and the field configurations for this mode areessentially the same as the m=0 mode in the case of the unifilar helix,with the exception that the impedance of the circuit has been increasedby the addition of the second helix. This bifilar arrangement, however,also allows the existence of an antisymmetric backward Wave mode thatcan interact with the electron beam and cause the tube to oscillate atlow values of beam current. The R-F electric field distribution alongthe bifilar helix for the case of the.

antisymmetric backward wave mode is illustrated by FIG. 4. In this modeof operation the two helices act similar to a two-wire transmission linein the TEM mode, and either the symmetric or antisymmetric mode canexist independently. Suppression of the antisymmetric mode, therefore,will not adversely affect the overall tube gain or elficiency.

. Since the electron beam can interact with the fields of thebackward-wave antisymmetric mode and cause the tube to oscillate, it isthe primary object of my invention to provide a tube of the typedescribed having a bifilar helix structure, the unique geometricconfiguration of which is particularly adapted to suppress saidantisymmetric mode. I propose, therefore, a slow wave structurecomprising two helically wound wire transmission lines interwound in abifilar manner. I further propose that one of said slow wave structuresbe fabricated from wire of substantially larger diameter than the othersaid structure. FIGURE 1 illustrates such an arrangement wherein slowwave structure 13 is interwound with slow wave structure 12. Therelative wire sizes are illustrated by d4 and d3 in FIGURE 2. Withrespect to the wire sizes, it is well known that if a wire is. verysmall there will be much electric and magnetic energy very close to thewire which is not associated with the desired field component. Also, ifa Wire is very large the internal diameter of the helix becomesconsiderably less than the mean diameter, and the space available forelectron flow is reduced. Therefore, in the present invention, wirediameters d3 and d4 are designed to be as large land as small,respectively, as is compatible with frequency and circuit parameters.

From an examination of FIGURES 2 and 5, it is obvious that, for thepurpose of antisymmetric backward Wave-mode suppression, mean diameterd1 of helix 12 should be small compared to mean diameter (12 of helix13. This will suppress beam interaction with the backward-wave modesince, due to the difference in mean diameters, the fields of theantisymmetric backward wave mode have a component directed away from thebeam and are partially shielded from it by the larger diameter helix.

Additional suppression of the antisymmetric backward wave mode isachieved by coating small wire helix 13 with some lossy substance suchas aquadag. While this will further attenuate the antisymmetric mode andpre vent oscillation from building up in the mode, it will also incursome loss in the symmetric mode. This loss in the symmetric mode can bekept to a minimum, however, by proper excitation and by optimizing theloss of small Wire helix 13.

The making of one helix of a smaller wire and a larger mean diameternecessitates that said helix be operated at a diiferent potential thanwould be required for the same focusing characteristics obtained whenboth helices are of the same size wire. This is an additional advantageof my invention, in the case of electrostatic focusing, if the potentialis made to correspond to the potential that exists on one of theeleectron gun electrodes.

Referring now to FIGURE 1 there is illustrated a traveling-wave tubeamplifier employing the principles of my invention. The traveling-wavetube thus illustrated comprises electron gun 20 and collector electrode15 at the respective ends of tubular vacuum envelope 11. Cooling fins 16of collector electrode 15 provide rapid heat dissipation. Electron gun20 is of conventional design and includes a heater, an emissive cathodebutton, a focusing electrode, and heater supply 22. Acceleratorelectrode 21 is arranged to direct a beam of electrons along the axis ofthe tube to collector 15. Large wire conductive helix 12 is supportedlongitudinally within vacuum envelope 11 by ceramic dowels 14. Ceramicdowels 14 have lossy coatings 17 disposed thereon for the purpose ofattenuating reflected energy of the forward mode, said lossy coatings 17being arranged in such a manner as to preclude any possible partialshort-circuit between helix turns. Helix 12 is terminated at its endnearer the cathode electrode by an arrangement for coupling to inputwaveguide 18, and its end nearer the collector electrode by anarrangement for coupling to output waveguide 19. In the present example,this is effected by providing the helix with extensions 27 and 28,respectively, saidextensions being proximate to said input and outputwaveguides. Small wire conductive helix 13 is interwound with helix l2and is insulated therefrom. Helix 13 is supported by dowels 14 in thesame manner as helix 12, and is made lossy over its entire length. Apower supply source, such as battery 23, is connected to providesuitable operating potentials for the various tube elements. Helix 13 ismade to operate at the same potential as cathode electrode 20, and avoltage difference between helix 13 and helix 12 of 2000 v. ismaintained.

Although the present invention has been described with particularreference to a certain specific embodiment, it will be understood thatthe invention is capable of still other forms of physical expression,and consequently is not limited to the specific disclosure, but only bythe scope of the appended claims.

I claim:

1. In a traveling wave amplifier tube, a slow wave structure comprisingfirst and second wire helices arranged in a bifilar relationship, saidfirst helix being fabricated of smaller diameter wire than said secondhelix, and a plurality of dielectric rods disposed externally adjacentto both said wire helices, and parallel to the common axis thereof, forthe purpose of supporting said slow wave structure.

2. In a traveling wave amplifier tube having a cathode and an anode, aslow wave structure comprising, in combination, first and second helicalwire transmission lines, each of said transmission lines beingelectrically isolated and interwoven on a common longitudinal axis in abifilar arrangement, said first transmission line being maintained atcathode potential, said second transmission line being maintained at ahigher electrical potential and fabricated of substantially largerdiameter wire than said first transmission line, and a plurality ofdielectric rods disposed externally adjacent to said transmission linesand parallel to the axis thereof for the purpose of supporting said slowwave structure in axial alignment.

3. In a traveling wave tube amplifier a slow wave structure comprisingfirst and second conductive wire helices, said first helix beingfabricated from substantially smaller diameter wire than said secondhelix and said first and second helices being interwound about a commonlongitudinal axis in alternating contiguous turns, said turns being ofequal outside diameter and having electrical isolation therebetween, anda plurality of dielectric rods disposed externally adjacent to both saidWire helices, and parallel to the common axis thereof, for the purposeof supporting said slow wave structure.

4. A slow wave structure as set forth in claim 3 wherein said firsthelix is of relatively high loss.

5. A traveling wave amplifier tube comprising a slow wave structurehaving input and output means and consisting of first and secondconductive wire helices, said first helix being fabricated fromsubstantially smaller diameter wire than said second helix and saidfirst and second helices being interwound about a common longitudinalaxis in alternating contiguous turns, said turns being of equal outsidediameter and having electrical isolation therebetween, a rigid envelopecomprising a series of parallel supporting rods and a straightcylindrical portion surrounding said slow wave structure coaxially andspaced radially therefrom, means to impress high frequency waves to beamplified upon said slow wave structure input means, an electron gunpositioned to project a beam of electrons along said slow wave structurein energy exchanging relationship with the high frequency wavestraveling thereon, a direct current source connected to said electrongun and to said helices, and adjusted to provide beam velocity andfocusing compatible with said slow wave structure and the high frequencywaves traveling thereon, and means for deriving said high frequencywaves in amplified form from said output means of said slow wavestructure.

References Cited by the Examiner UNITED STATES PATENTS 2,811,673 10/57Kompfner 31539.3 2,930,925 3/60 Weglein 3153.5 3,013,177 12/61 Minakovic315-,3.5

GEORGE N. WESTBY, Primary Examiner.

JOHN W. HUCKERT, Examiner.

1. IN A TRAVELING WAVE AMPLIFIER TUBE, A SLOW WAVE STRUCTURE COMPRISING FIRST AND SECOND WIRE HELICES ARRANGED IN A BIFILAR RELATIONSHIP, SAID FIRST HELIX BEING FABRICATED OF SMALLER DIAMETER WIRE THAN SAID SECOND HELIX, AND A PLURALITY OF DIELECTRIC RODS DISPOSED EXTER- 